Eddy current method for detecting a flaw in semi-conductive material

ABSTRACT

An eddy current probe, excitable at frequencies equal to or greater 7 mhz., includes a substantially planar, spiral-like, unilaminar coil mounted on an insulative substrate. A protective layer covers the coil and is adapted to present only a minimal mechanical barrier between the coil and a region of material under test. The coil is a continuous run of copper having an effective diameter no greater than 2 mils. The coil, substrate and protective layer are a printed circuit board. A method of detecting flaws is also disclosed.

BACKGROUND OF THE INVENTION

This invention relates to an apparatus and method for detecting flaws inthe surface and sub-surface region of a material by inducing an eddycurrent therein at a relatively high frequency.

The ability of an eddy current probe to detect flaws in the surface andsub-surface region of materials depends upon the resistivity of thematerial and the excitation frequency applied to the coil within theeddy current probe. The resistivity of a material provides aclassification of the material as conducting or semi-conducting, hence,a material having a relatively high value of resistivity is commonlyrecognized as semi-conducting in nature. To nondestructively test asemi-conducting material, a relatively high frequency is necessary toinduce an eddy current within the material. For example, to detect ahair line surface crack in a block of carbon, the eddy current probemust have its coil excited at a relatively high frequency, a frequencywhich approaches 6 megahertz.

The excitation frequency of the coil also affects the depth ofpenetration of the eddy currents in the material under test. As commonlyrecognized in the nondestructive testing art, the higher the frequencyof excitation, the lesser the depth of penetration within thesub-surface region of the material under test. If the depth ofpenetration in the above example is limited to approximately 35 mils,the coil must be excited at a frequency equal to or greater than 7megahertz.

High sensitivity, commercially available, eddy current probes have anupper frequency range limitation of 6 megahertz. An example of one highfrequency eddy current probe is the SPO2000 Probe manufactured by NortecInc. of Kennewick, Wash. These high sensitivity probes generally havefine wire wound about a small bobbin. To achieve the high sensitivity todetect the hair line surface crack in the carbon block example, the coiltherein must be excited by a signal approaching or exceeding 7megahertz. At that high frequency, the interwire capacitance between theturns in the coil generally acts as a capacitive shunt across the entirecoil. This shunt effectively shorts out the coil's output, hence theprobe no longer functions properly.

The high frequency required in the above example dictates a reduction inthe number of turns of the coil and an enlargement of the spacingbetween each turn. Other considerations require that the coil qualityfactor (Q) be greater than 1 and preferably be greater than 10. As iswell known in the art, the coil quality factor is related to the ratiobetween the energy stored in the coil and the energy dissipated in thecoil. The AC coil losses are functionally related to the energydissipated and are dependent upon the diameter of the wire. Forfrequencies on the order of 7 megahertz, the wire diameter must be lessthan or equal to 2 mils. If the frequency is increased to 10 megahertz,a wire having a diameter of 1.4 mils is recommended.

The construction of a coil with wire less than or equal to 2 mils isvery difficult due to the fineness and fragility of the wire. Anadditional factor to be considered is the geometry of the coil. In thecarbon block example above, the precise location of the hair linesurface crack is not known. Hence, the surface area to be covered by thecoil is relatively large in relation to the depth of penetration of theeddy current in the material. As recognized in the art, the geometricconfiguration of the windings of the coil is important for the faithfulreproduction of the coil. It is desirable to obtain at least two coilshaving similar levels of impedance. To meet the above requirements forthe carbon block example, i.e., small depth penetration by the eddycurrents, relatively large surface area, a relatively high excitationfrequency, windings having a diameter on the order of 2 mils, anduniform but relatively large spacing between each winding (to minimizeinterwire capacitance), the coil must be substantially planar. A planarcoil induces an eddy current in the carbon material over a large surfacearea and if the excitation frequency is high, at a relatively limiteddepth into the sub-surface region of the carbon block.

Experiments have shown that a fine wire, having a diameter of 2 mils, 44gage AWG, placed on a planar insulating surface, cannot be replicatedwith sufficient accuracy. Thus the coils constructed by this method hadimpedances varying from coil to coil by approximately 10%. The variationin impedances between these experimental coils is not acceptable whenthe coils are utilized to detect flaws in materials. It is known thatplanar, unilaminar coils have been utilized as inductors on printedcircuit boards. However, those printed circuit board inductors do notnecessarily generate uniform electromagnetic fields proximate theircoils.

OBJECTS OF THE INVENTION

It is an object of this invention to provide for an apparatus to detectflaws in the surface and sub-surface region of a semi-conducting orconducting material which possesses a high degree of sensitivity, andincludes a coil excitable at a frequency equal to or greater than 7megahertz.

It is another object of this invention to provide for an apparatus whichis reproducible such that similarly constructed coils have substantiallycomparable impedances when those coils are excited at the samefrequency.

It is a further object of this invention to provide for a coil which maybe placed over a cambered support surface.

An additional object of the present invention provides for an eddycurrent coil mounted on an insulative substrate and both are a printedcircuit board.

SUMMARY OF THE INVENTION

An apparatus, which detects flaws in the surface and sub-surface regionof a semi-conducting or conducting material, includes means forgenerating a uniform electromagnetic field normal to a substantiallyplanar, spiral-like, unilaminar coil mounted on an insulative substrate.A protective insulative layer covers the coil and is adapted to presentonly a minimal mechanical barrier between the coil and the region ofmaterial under test. In one embodiment, the coil includes a continuousrun of metal, such as copper, having an effective diameter no greaterthan 2 mils. In a further embodiment of the present invention, the coiland substrate is a printed circuit board.

The invention additionally includes means for detecting the impedance ofthe coil and providing a representative signal, means for establishing areference signal, means for placing the coil adjacent the region ofmaterial under test, and a comparison means. The comparison meansprovides an output signal indicative of the integrity of the region ofmaterial under test by relating the reference signal to the measuredimpedance signal.

In another embodiment of this invention, the unilaminar coil issubstantially planar but cambered and is mounted on an insulativesubstrate.

A method of detecting flaws in the surface and sub-surface region of thematerial includes the steps of: providing a continuous run of metal on asubstantially planar insulative substrate as a unilaminar coil,providing means for exciting said coil, establishing a reference signal,juxtapositioning the coil adjacent the region of material under test toinduce an electromagnetic field therein, and comparing a signalrepresentative of the impedance of the coil with the reference signal toprovide an output signal indicative of the integrity of the materialunder test.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed and distinctly claimed in the concluding portion of thespecification. The invention, however, together with further objects andadvantages thereof, may best be understood by reference to the followingdescription taken in connection with the accompanying drawings in which:

FIG. 1 illustrates a magnified, top view of an eddy current coil withwhich the invention may be practiced;

FIG. 2 i11ustrates a magnified side view, from the perspective ofsectional line 2--2' in FIG. 1, of the excited eddy current coil;

FIG. 3 illustrates a different geometric configuration of the eddycurrent coil;

FIG. 4 illustrates a greatly magnified side view of the eddy currentcoil as viewed from the perspective of sectional line 4--4' of FIG. 1;

FIG. 5 is a schematic of a simple electrical circuit incorporating theeddy current coil;

FIG. 6 illustrates one exemplary use of the invention herein;

FIG. 7 illustrates another utilization of the present invention with asubstantially planar but cambered unilaminar coil.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates generally to an eddy current coil which isoperated at a frequency equal to or greater than 7 megahertz therebyinducing an eddy current in a semi-conducting material at a relativelyminimal depth of penetration.

FIG. 1 illustrates the top view of eddy current coil 10 mounted on asubstantially planar insulative substrate 12. Coil 10 and substrate 12are part of means for generating a substantially uniform electromagneticfield extending normal to the surface of substrate 12. Coil 10 includesa continuous run of metal geometrically configured spiral-like onsubstrate 12. As used herein, the term "spiral-like" means a unilaminarcoil having one end centrally located and the run of metal completelysurrounding that centrally located end. The run of metal is wound aboutitself and has a second end spacially displaced from the centrallylocated end on the generally planar surface of the substrate. Hence, thespiral-like geometric configuration of the coil in FIG. 1 possessessquared corners, but nonetheless, coil 10 is considered spiral-like forthe purposes of this invention.

Coil 10 is a 14 turn coil composed substantially of copper having aneffective diameter of approximately 0.7 mils. It is to be understoodthat although the effective diameter of a run of copper in coil 10 is0.7 mils, the invention could utilize a run of copper, or other metalsuch as silver or platinum, having an effective diameter no greater than2 mils. Coil 10 is easily reproducible because coil 10 and substrate 12is a printed circuit board. Solder pad 14 is electrically connected tothe centrally located end of coil 10. Solder pad 16 is electricallyconnected to the spacially displaced outer end of coil 10. Individualsordinarily skilled in the art of printed circuit board technology willrecognize that coil 10, coil 10's geometry and the small effectivediameter of each run within coil 10 is readily reproducible. Further,experimental tests have shown that a printed circuit board utilizing arun of copper having an effective diameter of 0.7 mils thereon,configured as illustrated in FIG. 1, is readily reproducible and coilsconstructed in accordance with this invention have relatively matchingand comparable impedances within the desired operating range. Thematched impedances of the coils are desirable when the coils areutilized in eddy current probes.

FIG. 2 is a side view of the printed circuit board in FIG. 1 as viewedfrom the perspective of sectional line 2--2'. Insulative substrate 12 isillustrated as substantially planar with a substantially uniformelectromagnetic field 20 extending normal from its surface when coil 10is excited. Electric leads are coupled to solder pads 16 and 14respectively by means well known in printed circuit board technology.

FIG. 3 illustrates a different geometric configuration of coil 10wherein the coil is a circular rather than rectangular spiral-like runof metal.

FIG. 4 is a magnified side view of three windings of coil 10 as viewedfrom the perspective of sectional line 4--4' in FIG. 1. Three distinctturns 30, 32, and 34 of the run of metal of coil 10 are illustrated inFIG. 4. To protect the coil from the ambient environment, a protectiveinsulative layer 36 substantially covers the coil. Layer 36 is adaptedto present only a minimal mechanical barrier between the coil and theregion of semi-conducting or conducting material under the flawdetection test.

In the example under consideration herein, where a hair line surfacecrack is sought to be detected in the surface and sub-surface region ofa block of carbon, and the sub-surface depth of penetration of the eddycurrent is limited to approximately 35 mils, the following dimensionsfor coil 10 and substrate 12 are utilized. The fiber glass insulativesubstrate 12 is approximately 62 mils along dimension "a" in FIG. 4. Asstated earlier, the effective diameter of the run of copper is 0.7 milsas grossly illustrated as dimension "b". Each winding is spaced apartapproximately 8 mils as shown as dimension "c" from the centerline ofwinding 30 to the centerline of winding 32. Dimension "d" is the depthof layer 36 and is approximately 1 mil.

Those of ordinary skill in the art will recognize that the effectivediameter of the run of copper and the spacing between each winding ofthe eddy current coil is based upon such factors as the resistivity ofthe material under test, the desired depth of penetration for the eddycurrent, and the liftoff distance between the effective centers of thewindings and the surface of the material under test. Calculations basedupon these parameters will determine the appropriate effective diameterof each winding as well as the spacing between each winding. It is to berecognized that the depth of layer 36, or dimension "d", is based uponmany factors such as the abrasive characteristics of the surface ofmaterial under test, the effective diameter of the run of metal, and theambient environment in which the eddy current probe is operated. It isto be understood the dimensions and compositions described herein areillustrative and disclose one working embodiment of the invention.

Although substrate 12 is illustrated as having a relatively planarsurface 40, that surface may be slightly curved or cambered. Thecritical limitation to the amount of camber of surface 40 is based upona calculation of the uniformity of the electromagnetic field extendingrelatively normal the coil 10 mounted on surface 40. The term "cambered"as used herein refers to a surface which is slightly concave or convexrather than perfectly flat.

Although substrate 12 is discussed herein as being a substantially solidinsulative body, the substrate may be flexible. For example, thesubstrate may be composed of a mylar plastic or similar insulativematerial having a dimension a between 0.25 and 30 mils. A coil affixedto such a substrate would produce an eddy current coil which isrelatively flexible and which could be placed on a suitable support. Thesubstrate may also be a solid body of saffire.

FIG. 5 is a schematic of a simple electrical circuit including coil 10.Circuit 42 is part of the means for generating the electromagnetic fieldextending from coil 10. Circuit 42 includes a balanced impedance bridgecircuit 44 which detects the impedance of coil 10. Coil 10 is one leg ofbridge circuit 44 as is coil 46. Coil 46 is substantially similar tocoil 10 but coil 46 is utilized as a reference for circuit 44.Adjustable impedance means 48 and 50 provide the two remaining legs ofbridge circuit 44. Impedance means 48 and 50 are adjustable to balancebridge circuit 44 and an alternating current signal generator 52provides power and an excitation signal to bridge circuit 44.

As is well recognized in the art of nondestructive testing, coil 10 isplaced or juxtapositioned adjacent the region of material under test toinduce an electromagnetic field in that region. The distance between theeffective centers of the windings of the coil and the surface ofmaterial under test is generally recognized as the "liftoff" distance.As used herein, the term "region" refers to the combination of thesurface area and sub-surface adjacent portions of the area of thereferenced material. Coil 46 is substantially similar to coil 10 and theoperation and establishment of a balanced bridge circuit 44 is wellrecognized in the art. Coil 46 is normally placed in relatively freespace when coil 10 is so juxtaposed proximate an unflawed region ofmaterial, and impedance means 48 and 50 are adjusted such that bridgecircuit 44 is either balanced or slightly unbalanced. An output signalat terminals 54 provides an indication of the integrity of the region ofmaterial under test, i.e., the output signal provides an indicationwhether the material is flawed or unflawed. Alternatively, coil 46 maybe placed adjacent an unflawed region of material. When coil 10 isadjacent a region of material under test in such configuration, bridgecircuit 44 generates a signal representative of the integrity of theregion.

As is well recognized in the art, a reference signal must be establishedto determine whether the region of material under test is or is notflawed. One recognized procedure to establish this reference signal isto place coil 10 adjacent an unflawed region of material and balancecircuit 44. A second recognized procedure is to establish a referencesignal level based upon some theoretical calculations relating to coil10, the frequency of excitation, and the material under test. Both ofthese procedures generate a reference signal corresponding to theimpedance of coil 10 when the coil is adjacent an unflawed region ofmaterial under test. A third method, specifically related to the carbonblock example, is to place coil 10 over an unflawed region substantiallysimilar to the region of material to be tested for hair line cracks. Inthis fashion, the reference signal may reflect the influence of holes,copper pigtails or whatever in the subsurface region under test. Afourth method places coil 46 over an unflawed region and coil 10 overthe region under test.

In one well known method of operation, bridge circuit 44 is adjusted tobe slightly unbalanced when coil 10 is adjacent an unflawed region ofmaterial. As described herein, the term "slightly unbalanced" issynonomous with "substantially balanced". The output signal at terminals54, when bridge circuit 44 is slightly unbalanced or substantiallybalanced, is a sinusoidal signal having a small amplitude. Thereafter,coil 10 is placed adjacent the region of material under test to inducean eddy current in the surface and sub-surface region of that material.If the region is unflawed, or the integrity of the material ismaintained, the small amplitude sinusoidal output signal will besubstantially unchanged. However, if the region of material is flawed,due to a crack in the material, an unexpected change in the compositionof the material, or an unexpectedly thin layer of material (ascontrasted with a thick layer) over a known base material, the outputsignal from bridge circuit 44 will reflect a substantially differentsinusoidal output signal at terminals 54. In this manner, the outputsignal at terminals 54 supplied to a well known comparison meansprovides an indication of the integrity of the material under test byrelating the reference signal to the output signal which is a signalrepresentative of the impedance of coil 10 when the coil is adjacent theregion of material under test.

FIG. 6 illustrates one type of material subject to testing by the eddycurrent probe described herein. Block 60 consists essentially of carbon.A pair of holes are drilled diagonally into block 60 and copper pigtails62 and 64 are mechanically tamped into the two holes as illustrated inFIG. 6. In this example, block 60 has a thickness of approximately 205mils and the diameter of each hole is approximately 128 mils. Oneparticular use of block 60 is as a carbon brush, the carbon typically inthe form of graphite, in certain dynamoelectric machines. The copperpigtails provide an electrical connection between carbon block 60 andthe associated circuitry of the dynamoelectric machine. Nondestructivelytesting the surface area and sub-surface region immediately adjacent thecopper pigtails determines whether a hair line crack has developedduring the mechanical insertion of the pigtails 62 and 64 into block 60.A crack 66 is present due to pigtail 64's insertion. The surface areaunder test in this example is 1/2" by 1/8" as illustrated as dimensions"e" and "f" respectively in FIG. 6. Due to surface area "e"×"f", asomewhat rectangular but spiral-like eddy current coil illustrated inFIG. 1 is utilized.

Hair line crack 66 affects the eddy currents induced in the surface andsub-surface region of block 60 when coil 10 is juxtaposed to area"e"×"f" thereby altering the output signal at terminals 54 of the bridgecircuit 44. An electronic circuit, associated with circuit 42, amplifiesand filters the output signal to determine the presence of crack 66.

Since the holes for receiving pigtails 62 and 64 are respectivelycentered on the edge of block 60 as shown in FIG. 6, there are minimallyapproximately 38.5 mils of block 60 between copper pigtails 62 and 64when inserted into cooperating holes and the upper surface of block 60.It is desired to determine the integrity of block 60 after insertingpigtails 62 and 64. In order to avoid interference with and/ordistortion of measurements by the conductive copper of pigtail 64 whenattempting to detect crack 66 above pigtail 64, it is necessary to limitthe depth of penetration of eddy currents induced in block 60, forexample to a depth of 35 mils, to inhibit induction of eddy currents inpigtail 64. A similar depth limitation is required when the surface andsub-surface region above pigtail 62 is to be examined.

FIG. 7 illustrates another utilization of the eddy current coil herein.Coil 80 is substantially planar but cambered to match cambered surface82 of structure 84. As illustrated, the coil is slightly concave togeometrically approximate the slightly convex surface 82. Coil 80 issupported by insulative support structure 86 which is similarlycambered. As is well recognized in the art, to obtain accuratenondestructive testing results, the liftoff distance between theeffective centers of the windings of the eddy current coil and thesurface of the material under test must be maintained at a relativelyconstant and acceptable level. An eddy current coil constructed with aflexible insulative substrate, as described herein, could be placed on aslightly cambered support structure, such as structure 86, to maintainthat liftoff distance between the contoured surface 82 of a structure 84under test. Accordingly, the contour of support 86 could be altered tomatch the contour of surface 82 thereby maintaining an acceptablelift-off distance.

As is well recognized in the art, eddy current probes can be utilized todetermine cracks in materials, inclusions in materials, to determine thethickness of a particular conductive or semi-conductive materialdisposed as a layer on a base material, and to generally determine thestructural and metallurgical integrity of a host of materials. Theinvention herein is not intended to be limited by the specific usesdescribed herein. Also, it is recognized by those of ordinary skill inthe art that many electronic circuits are capable of detecting theimpedance of an eddy current coil, generating a reference signal, andcomparing the reference signal to the impedance of the coil when thecoil is adjacent a region of material under test. The simple electroniccircuit described herein, in association with the eddy current coil, isnot meant to limit the scope of this invention. The claims appended tothis specification are meant to encompass the modifications describedherein and those modifications apparent to individuals of ordinary skillin the art.

We claim:
 1. A method for detecting a flaw in a surface and/orsub-surface region of a semi-conductive material, the semi-conductivematerial including a conducting material disposed in the sub-surfaceregion at a predetermined distance from the surface of thesemi-conductive material, comprising the steps of:providing a continuousrun of metal configured spiral-like on a substantially planar insulativesubstrate as a unilaminar coil; exciting said coil to produce anelectromagnetic field extending from said coil, the electromagneticfield for inducing eddy currents in the surface and/or sub-surfaceregion of the semi-conductive material when said coil is positionedadjacent said semi-conductive material; establishing a reference signalcorresponding to the impedance of said coil when said coil is adjacentan unflawed region of said material; juxtapositioning said coil adjacentthe surface and/or sub-surface region of the semi-conductive material toinduce an electromagnetic field therein, while inhibiting induction ofeddy currents in the conducting material; and comparing a signalrepresentative of the impedance of said coil during saidjuxtapositioning step with said reference signal to provide an outputsignal indicative of the integrity of said surface and/or sub-surfaceregion of the semi-conductive material, whereby the flaw in the surfaceand/or sub-surface region of the semi-conductive material may bedetected.
 2. A method for detecting a flaw in a surface and/orsub-surface region of a semi-conductive material, the semi-conductivematerial including a conducting material disposed in the sub-surfaceregion at a predetermined distance from the surface of thesemi-conductive material, comprising the steps of:providing a continuousrun of metal configured spiral-like on a substantially planar butcambered flexible insulative substrate as a uilaminar coil; excitingsaid coil to produce an electromagnetic field extending from said coil,the electromagnetic field for inducing eddy currents in the surfaceand/or sub-surface region of the semi-conductive material when said coilis positioned adjacent said material; establishing a reference signalcorresponding to the impedance of said coil when said coil is adjacentan unflawed region of said material; juxtapositioning said coil adjacentthe surface and/or sub-surface region of the semi-conductive material toinduce an electromagnetic field therein, while inhibiting induction ofeddy currents in the conducting material; and comparing a signalrepresentative of the impedance of said coil during saidjuxtapositioning step with said reference signal to provide an outputsignal indicative of the integrity of said surface and/or sub-surfaceregion of the semi-conductive material, whereby the flaw in the surfaceand/or sub-surface region of the semi-conductive material may bedetected.
 3. The method as in claim 1 or 2, wherein said metal issubstantially copper and said coil and said substrate is a printedcircuit board.
 4. The method as in claim 1 or 2, wherein excitingincludes coupling an excitation signal at a frequency equal to orgreater than 7 megahertz to said coil.
 5. The method as in claim 4,wherein the step of establishing a reference signal includes using asubstantially balanced impedance bridge circuit having the coil as oneof its legs.
 6. The method as in claim 1 or 2, wherein saidsemi-conductive material consists essentially of carbon and said flaw tobe detected is a harline crack in said surface and/or sub-surface regionof said semi-conductive material, and further wherein a copper pigtailis disposed in said semi-conductive material at the predetermineddistance from the surface of the semi-conductive material.
 7. The methodas in claim 6, wherein the step of inhibiting includes exciting saidcoil at a frequency high enough to inhibit induction of eddy currents inthe copper pigtail.
 8. The method as in claim 6, wherein the carbon isin the form of graphite.